🐛 Ensure that Optional[list] values work correctly with callbacks

tests/test_others.py

@@ -144,6 +144,32 @@ def main(name: str = typer.Option(..., callback=name_callback)):
     assert "value is: Camila" in result.stdout
 
 
+def test_callback_4_list_none():
+    app = typer.Typer()
+
+    def names_callback(ctx, param, values: typing.Optional[typing.List[str]]):
+        if values is None:
+            return values
+        return [value.upper() for value in values]
+
+    @app.command()
+    def main(
+        names: typing.Optional[typing.List[str]] = typer.Option(
+            None, "--name", callback=names_callback
+        ),
+    ):
+        if names is None:
+            print("Hello World")
+        else:
+            print(f"Hello {', '.join(names)}")
+
+    result = runner.invoke(app, ["--name", "Sideshow", "--name", "Bob"])
+    assert "Hello SIDESHOW, BOB" in result.stdout
+
+    result = runner.invoke(app, [])
+    assert "Hello World" in result.stdout
+
+
 def test_completion_argument():
     file_path = Path(__file__).parent / "assets/completion_argument.py"
     result = subprocess.run(

typer/main.py

@@ -640,10 +640,10 @@ def convertor(value: Any) -> Any:
 
 def generate_list_convertor(
     convertor: Optional[Callable[[Any], Any]], default_value: Optional[Any]
-) -> Callable[[Sequence[Any]], Optional[List[Any]]]:
-    def internal_convertor(value: Sequence[Any]) -> Optional[List[Any]]:
-        if default_value is None and len(value) == 0:
-            return None
+) -> Callable[[Optional[Sequence[Any]]], Optional[List[Any]]]:
+    def internal_convertor(value: Optional[Sequence[Any]]) -> Optional[List[Any]]:
+        if value is None or len(value) == 0:
+            return default_value
         return [convertor(v) if convertor else v for v in value]
 
     return internal_convertor